Composition for removing scale

ABSTRACT

Alkaline earth metal scales, particularly barium sulfate and strontium sulfate, and silica-based scales, particularly potassium fluorosilicate, are removed from industrial processing equipment, tubular goods, and other scaled materials with a composition comprising sufficient potassium base to provide an overall system pH of between about 12.0 to about 14.0, a polyaminocarboxylic acid, preferably EDTA, a hydroxycarboxylic acid synergist, a pH buffering agent, a wetting agent, and optionally a sodium base.

This application is a divisional application under 37 C.F.R. §1.60 ofapplication Ser. No. 08/282,883, filed Jul. 29, 1994, now abandoned.

BACKGROUND OF THE INVENTION

Scales are a problem in the production of oil. Effluents, such as watercontaining alkaline earth metal cations, such as barium, strontium,calcium, and magnesium, along with anions, such as sulfate, bicarbonateand carbonate, are often encountered in producing fields. Whencombinations of these anions and cations are present in sufficientconcentrations, precipitation occurs, forming water insoluble scales.These water insoluble scales are otherwise known as alkaline earth metalscales. Some alkaline earth metal scales are calcium carbonate, bariumsulfate, and strontium sulfate.

Sometimes, in oil-producing formations, silica-based scales are alsoformed. Like the alkaline earth metal scales, these scales arewater-borne and highly insoluble. The most frequently encounteredsilica-based scale is potassium fluorosilicate.

While some scales are amenable to acid treatment, such as hydrochloricacid and hydrofluoric acid (otherwise known as "mud acid"), other scalessuch as barium sulfate, strontium sulfate, and potassium fluorosilicateare difficult, to impossible to dissolve or break up. These scales canprecipitate out in a variety of places, such as in the reservoir, orproducing formation, of a production well. They may form in the tubularsof a production well, as well as in pipelines, heater treaters, or otherequipment downstream from the wellhead. Scales may also form ininjection wells, in water flood plants, in filters, or anywhere variouswaters come together.

While alkaline earth metal scales and silica-based scales have beenproblematic in oil production, radioactive scales have become a veryexpensive problem. Naturally occurring radioactive material ("NORM"),such as the radium isotopes 226 and 228, have a strong affinity forthese scales. As scale begins to form on tubing, as well as on producedsand or other particles in the well, the radium isotopes 226 and 228become entrained within the scale matrix. Therefore, produced or removedsolids from wells may exceed radiation levels set forth by EPA fordisposal of nonhazardous oil field waste ("NOW").

While natural production of water and fluids from a well bore maycontain the radium isotopes 226 and 228, these isotopes are present atsuch small concentrations that they do not pose a radiation problem.Radiation only becomes a problem when these isotopes becomeconcentrated, as in scale deposits.

Because scales such as barium sulfate, strontium sulfate and potassiumfluorosilicate are so resistant to dissolution, the traditional means ofcleaning tubulars, pipelines, heater treaters, etc. has been tophysically remove the scales. However, upon removal of the scale, theoperator may have large volumes of solids contaminated with radioactivematerial. Currently, oil production companies and others have largeinventories of such solids containing alkaline earth metal scales withNORM entrained throughout. Present disposal methods and/or regulationsrequire expensive remedies.

While commercial chemicals exist that are slowly effective for partiallydissolving alkaline earth metal scales, none of the currently availablechemicals produce highly efficient or rapid rates of scale dissolution,particularly with barium sulfate and strontium sulfate. Furthermore,current methods for removing silica-based scales from oil formations,such as pure base treatment, are only minimally effective due to thehighly destructive action of the chemicals to the oil formation itself.

What is needed is a chemical which quickly and effectively facilitatesdissolution of alkaline earth metal scales and removal of silica-basedscales. If the chemical is used down-hole, high production can berestored to a well. If the chemical is used for surface treatment ofcontaminated scale, the radium isotopes entrained in the scale can bereturned into solution and the solids, such as sand and other material,can be rendered non-hazardous oil field waste for conventional wastedisposal.

SUMMARY OF THE INVENTION

The present invention is directed to novel scale-removing compositionsand methods of their use. The compositions of the present invention areuseful in dissolving alkaline earth metal scale deposits and removingsilica-based scale deposits, at markedly higher speeds and efficienciesthan prior scale-removing compositions. The present invention isparticularly useful in removing and/or dissolving scale deposits formedin industrial process units and equipment as well as dissolving scalematerials which have been removed and collected from such operations.The advantages of the present invention are especially appreciated inoil-related applications such as down-hole drilling and oil recoveryoperations from subterranean formations. For example, the compositionsof the present invention may be applied to well units, boilers, heatexchanger, storage vessels, valves, pipelines, tubular goods, and otherprocessing equipment exposed to scale-forming conditions in the oilfield and elsewhere. In addition, the compositions of the presentinvention may be applied to scaled materials such as sand collected fromoil processing equipment.

Typically, scale deposits form on surfaces exposed to mineral-containingeffluents such as water. The minerals usually comprise cations such aspotassium, barium, strontium, calcium and magnesium in combination withsulfate, bicarbonate, or carbonate anions. In some undergroundformations, silica-based anionic moieties are also encountered. Insufficient concentration, these cationic and anionic moieties areprecipitated from solution and deposited as water-insoluble scale.

As these scales are formed, naturally occurring radioactive materials("NORM") present in the water, such as radium 226 and 228 isotopes, mayalso become entrained in the mineral deposits. These radioactive scalesare commonly associated with oil field operations.

According to the present invention, scale deposits such as thosedescribed above are effectively dissolved and/or removed by contact withan aqueous solvent of a polyaminocarboxylic acid chelant in an highlyalkaline environment produced by the addition of a potassium basewherein a pH buffering agent, an hydroxycarboxylic acid synergist, awetting agent, and optionally a sodium base are included in the solventto accelerate the scale removal process. The buffering agent, synergist,wetting agent, and optional sodium base of the present invention permitintimate contact between the solvent and the scale such that thechelating agent is allowed to remove and sequester the cationiccomponents of the mineral deposits quickly and efficiently. Thus, thepresent invention permits scale dissolution and/or removal at rates muchfaster than heretofore possible.

DETAILED DESCRIPTION OF THE INVENTION

In forming the scale-dissolving compositions of the present invention,it is preferred that the chemical agents be combined in sufficientconcentration to form an undiluted, minimally active stock solution.Such a stock solution is preferred since it facilitates handling andstorage of the reagent mixture until such time as the composition isrequired for scale-cleaning treatment. Thereafter, the stock solutionmay be rendered "active" upon demand simply by diluting a measuredvolume of the stock solution with equal parts by volume of water. By"active" it is meant that the diluted stock solution would fully embodyall of the advantages of the present invention.

In preparing the stock solution, the polyaminocarboxylic acid chelatingagent preferably comprises ethylenediaminetetraacetic acid (EDTA).Sufficient chelant should be added such that the chelant concentrationin the stock solution is in the,range of about 0.5M to about 2.0M. Thisrange provides for maximum efficiency and economy of thescale-dissolving compositions produced in accordance with the presentinvention.

In addition to the chelant, the present scale-dissolving compositionsalso contain a potassium base, preferably potassium hydroxide, which isused to create an alkaline environment for the stock solution in therange of about 12.0 to about 14.0. The combination of the potassium basewith the acid chelant results in the formation of stable potassiumsalts.

In terms of weight percent, the amount of potassium hydroxide (KOH)required to provide a stock solution pH between 12.0 and 14.0 may rangebetween about 45% to about 55% of the final stock solution weight.Furthermore, in the interest of economy, it is preferred that technicalgrade potassium hydroxide solutions be used in the scale-dissolvingcompositions of the present invention. In particular, potassiumhydroxide reagents comprising approximately 45% KOH are preferred. Suchreagents are readily available commercially.

In addition to the potassium base, a sodium base, preferably technicalgrade sodium hydroxide (NaOH), may be added to the compositions of thepresent invention for use in treatment of silica-based scales, such aspotassium fluorosilicate, which typically form in undergroundreservoirs. When used in accordance with the present invention, thesodium hydroxide aids in the production of a potassium fluorosilicateslurry which does not degrade or otherwise destroy the surroundingsubterranean formation. Thus, the slurry may be pumped out of down-holesites for further treatment by any means known to those skilled in theart. Preferably, the sodium base is added in sufficient quantity tocomprise between about 4% to about 6% by weight of the final stocksolution, and preferably 5% by weight.

As a carrier for the chemicals of the present invention, water ispreferably added in sufficient quantity to comprise between about 4% toabout 5% by weight of the final stock solution. Generally, addition ofwater in excess of 5% is undesirable since it may result in premature"activation" of the scale-dissolving composition, as described above.

Another constituent of the scale-dissolving compositions of the presentinvention is the pH buffering agent. The pH buffering agent preferablycomprises a potassium salt, and more preferably potassium carbonate. Theadditional potassium cations furnished by the reagent buffer thealkaline chelant solution according to the common ion principle ofbuffering. As a result of the buffering action provided by the potassiumsalt, the present invention may be used in both acidic and alkalineenvironments without jeopardizing the overall alkalinity of thescale-dissolving composition.

It is preferred that the pH buffering agent be added in sufficientconcentration to comprise between about 4% to about 6% by weight of thefinal stock solution. Furthermore, it is preferred that the potassiumsalt be added in anhydrous form to prevent unnecessary dilution of thestock scale-dissolving composition.

In addition to the pH buffering agent, a synergist comprised of ahydroxycarboxylic acid, or salt thereof, is added to thescale-dissolving compositions of the present invention. While notdesiring to be bound to any particular theory, it is believed that thehydroxycarboxylic-based synergist aids in chelation of divalent cationssuch as barium, strontium, calcium, and magnesium. The synergists of thepresent invention are also particularly well-suited for use in highlyalkaline environments, in comparison to other organic synergists,permitting faster rates of scale dissolution than heretofore achievable.

Within the class of hydroxycarboxylic-based synergists, sodiumglucoheptonate is preferred, owing to its relative abundance andinexpensive cost. According to the present invention, the sodiumglucoheptonate is preferably added in an amount sufficient to comprisebetween about 10% to about 15% by weight of the final stock solution,and preferably around 13% by weight of the final stock solution.Moreover, in forming compositions for use in the treatment of alkalineearth metal scales, it is preferred that the sodium glucoheptonate berendered "optically inactive" prior to its introduction to thescale-cleaning stock solution. By "optically inactive" it is meant thatboth the "A" and "B" isomers of the sodium glucoheptonate areselectively removed. However, the "B" isomer of sodium glucoheptonate ispreferably maintained in compositions used for the treatment ofsilica-based scales. However, the present invention is not limited tosodium glucoheptonate synergists of such form.

To improve the accessibility of the compositions of the presentinvention to scaled surfaces, a wetting agent may also be added to thestock solution. The wetting agent may comprise a commercially availablesurfactant or other synthesized reagent containing methanol and sodiumchloride. Such wetting agents are capable of improving ionic exchangebetween the aqueous compositions of the present invention and scaledsurfaces containing non-polar, organic coatings, such as oil. Thus,wetting agent additives are especially preferred in compositions used totreat oil field scales.

Of the commercially available surfactants, Miranol Jem®, a proprietaryproduct of Rhone-Poulenc Chemical Company, is preferred due to itsmicellar structure and high efficacy in promoting scale dissolution inoily environments. The surfactant is preferably added in sufficientquantity to comprise between about 0.5% to about 1.5% of the totalweight of the stock solution.

Upon combination of the polyaminocarboxylic acid chelant, potassiumbase, pH buffering agent, synergist, wetting agent, and optionally asodium base in accordance with the present invention, the compositionmay be stored until such time as the stock solution is required forscale-removal treatment. Prior to treatment, the composition may be"activated" by diluting a measured volume of the solution with equalparts by volume of water. Of course, further dilution may be desired foreconomy, depending on the particular treatment parameters such as thedegree of scale removal required.

The "activated" compositions of the present invention may be applied toprocessing equipment, tubular goods, and other products to dissolveand/or remove scale deposits. The "activated" compositions may also beapplied to scaled materials such as sand recovered from oil processingequipment. Presently, some sands are precluded from landfill disposaldue to the presence of radioactive isotopes (i.e., radium 226 and 228)in the scale matrix. By treating these scaled sands with thecompositions of the present invention, the scale deposits may bedissolved or otherwise removed and the radioactive isotopes liberatedinto solution, leaving clean, non-radioactive sand for disposal inconventional landfill sites. The resultant liquid or slurry may then bedisposed of as conventional produced or treatment fluids.

Prior to above-ground scale-dissolution treatment, it is preferred thatthe composition be heated to between approximately 140° F. to 200° F.Under these conditions, complete scale dissolution or solubilization istypically achieved in between one to 200 minutes. Of course, mechanicalagitation of the solution and/or the scaled material will tend topromote scale dissolution or solubilization and reduce the overalltreatment time. Applicant incorporates by reference the copendingapplication filed by Inventor Robert D. Tate on Jul. 29, 1994 titled"Method for Treating Scale" which describes particular treatmentequipment which may be used to dissolve scale from oil field sands andother materials using the compositions of the present invention.

In subterranean scale-dissolution treatment, pre-heating is usually notrequired due to the high underground temperature and pressure. Thetreatment fluid may be placed in contact with the down-hole scale by anymethod generally used for a typical acid treatment. For example, thetreatment fluid may be bull-headed, spotted with tubing, spotted withcoiled tubing, etc.

Once the treatment fluid is in contact with the scale, it should not beleft to simply sit static throughout the treatment. Periodically, thefluid should be agitated to allow fresh, unspent chemicals to contactthe remaining mineral deposits to ensure maximum scale dissolution.Agitation may be accomplished simply by opening the well up and allowingsome fluid to flow up through the tubing. At the other extreme,agitation may be provided by a recirculating coiled tubing jet washwhich continually recirculates the treatment fluid across the scale.

The described cleaning procedure works equally well for scale containedin down-hole production equipment as well as scale that has formed inthe reservoir. It is preferred that any down-hole treatment be performedover a 24 hour shut-in of the production line, with periodic agitation.However, depending on the amount of scale build-up, a shorter treatmenttime may be used.

We claim:
 1. A method for removing silica-based scale deposits fromprocessing equipment or scale-bearing sands comprising contacting thescale with an aqueous solution formed from a stock solution, said stocksolution having sufficient potassium base to provide a stock solution pHof about 12.0 to about 14.0 and comprising a polyaminocarboxylic acidchelant or salt thereof in an amount sufficient to create a chelantconcentration of about 0.5M to about 2.0M in the stock solution prior todilution to create said aqueous solution, sodium glucoheptonatesynergist comprising about 10% to about 15% of the weight of the saidstock solution, a pH buffering agent in sufficient concentration tocomprise about 4% to about 6% of the weight of said stock solution, anda wetting agent in sufficient quantity to comprise about 0.5% to about1.5% of the weight of said stock solution.
 2. The method of claim 1wherein said silica-based scale is potassium fluorosilicate.
 3. Themethod of claim 1 wherein said stock solution further comprises a sodiumbase in an amount between about 4% to about 6% weight.
 4. The method ofclaim 1 wherein said processing equipment comprises industrialprocessing equipment.
 5. The method of claim 1 wherein said processingequipment comprises oil field processing equipment.
 6. The method ofclaim 1 wherein said processing equipment comprises tubular goods. 7.The method of claim 1 wherein said deposits are removed fromscale-bearing sands.
 8. A method for dissolving alkaline earth metalscale deposits from processing equipment or scale-bearing sandscomprising contacting the scale with an aqueous solution formed from astock solution, said stock solution having sufficient potassium base toprovide a stock solution pH of about 12.0 to about 14.0 and comprising apolyaminocarboxylic acid chelant or salt thereof in an mount sufficientto create a chelant concentration of about 0.5M to about 2.0M in thestock solution prior to dilution to create said aqueous solution, asodium glucoheptonate synergist comprising about 10% to about 15% of theweight of said stock solution, a pH buffering agent in sufficientconcentration to comprise about 4% to about 6% by weight of said stocksolution and a wetting agent in sufficient quantity to comprise about0.5% to about 1.5% of the weight of said stock solution.
 9. The methodof claim 8 wherein said processing equipment comprises industrialprocessing equipment.
 10. The method of claim 8 wherein said processingequipment comprises oil field processing equipment.
 11. The method ofclaim 8 wherein said processing equipment comprises tubular goods. 12.The method of claim 8 wherein said deposits are dissolved fromscale-bearing sands.